DDWFTTW - Directly Downwind Faster Than The Wind

Discussion in 'Propulsion' started by Guest625101138, Jan 4, 2009.

  1. kerosene
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    kerosene Senior Member

    Congratulations!.
     
  2. ThinAirDesigns
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    ThinAirDesigns Senior Member

  3. Windmaster
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    Windmaster Senior Member

    Hi
    Nowhere in the above article or anywhere on the Faster than the Wind website can I see mention of Jack Goodman - a member of the Amateur Yacht Research Society who, with his downwind faster than the wind model (on youtube here: http://www.youtube.com/watch?v=aJpdWHFqHm0) showed that it was possible.

    I don't think Jack was the originator of the idea (which has been talked about on AYRS channels for many years) but he was the first to build a model, film it, and demonstrate that it worked.

    I am willing to be corrected on this if I have got the timescale wrong, but it seems a shame that in all the ballyhoo, the first demonstrator of this concept should be forgotten and not given any mention.
     
  4. ThinAirDesigns
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    ThinAirDesigns Senior Member

    I have no control of the article, but Jack is featured prominently on the faster than the wind website -- check out the very third post:

    http://www.fasterthanthewind.org/2009/10/machinist-and-internet-hoax_03.html

    You are correct, Jack was not the originator, nor the first to build one (see the very first post on our project blog)

    By all accounts so far it was first done in the the '60s by a team of engineers from Douglas Aircraft, but even they weren't the ones who came up with the idea and as they did it to settle a bet between friends, they didn't document their exploits in a way that satisfied very many critics.

    AMO Smith (google him), the Supervisor of Aerodynamics Research and Chief Aerodynamics Engineer at Douglas and one of his wind tunnel engineers, Dr. Andrew Bauer discovered the idea in a paper presented by a midwestern student who was was angling for a summer internship.

    Unfortunately, no one remembers who the student was (they didn't get the intern position) so credit for the actual invention will apparently remain nebulous for all time.

    Bauer believed the student was correct and AMO believed DDWFTTW to be impossible. They made a bet and Bauer assembled a small team, built and sucessfully tested the device. AMO paid off on his bet and by all acounts it was near 40 years before anyone physically tried it again (Goodman).

    [​IMG]


    We give Jack HUGE credit everywhere we go and are in regular phone and email contact with him to this day. Check out the video that we did when we were trying to get this on MythBusters:

    http://www.youtube.com/watch?v=_fBDcchw5nw

    In history if we are remembered in any way with regard to DDWFTTW, it will be because we truly were the first group who documented it well enough to bring most rational people around. Before us most folks still called Goodman and Bauer crackpots and hoaxers -- now it's only the moonshot denier types who will never be convinced.

    JB
     
  5. spork

    spork Previous Member

  6. Windmaster
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    Windmaster Senior Member

    A lot of fuss about something obvious?

    You don't have to have a degree in maths or use any maths at all to see how it works.
    It's really just common sense. The craft doesn't care if the ground is moving or the air is moving. (How could it know?) It's the relative movement between the two mediums that counts.
    So it can make progress over the ground, driven by the wind, or it can make progress through the air driven by the ground.

    I'd be interested to hear from anyone what part of the above statement they don't understand.

    There is nothing amazing about it. The most amazing thing is that so many people can't understand it!
     
  7. spork

    spork Previous Member

    I think most people feel things should act more like a leaf blowing in the wind - but dragging on the ground. It will go a little slower than the wind over the ground, and a lot slower than the ground under the wind. Because our cart goes about 3X as fast as the wind, they claim it violates the laws of conservation of energy and momentum - but they never explain why that would be.
     
  8. zawy
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    zawy Junior Member

    This post explains the exact difference between powering a DDWFTTW vehicle with wind or treadmill. The "executive summary" is that there is no difference if there are no friction losses in the wind-mode. The energy from the treadmill motor is a gift to the system that overcomes some of the friction loses that would otherwise penalize the vehicle in wind-driven use.

    For Vc>Vw, Dr. Bauer and Dr. Drela equations are perfectly accurate only for the treadmill. The treadmill has the following distinct acceleration advantage over the wind-car changing by the factor: (following Bauer's notation in eq 13):

    [L(n-1)+L^2)]/[L(n-1)+(v1^2-v2^2)/Vw^2]

    n = Vc/Vw
    L= v/Vw
    v = v1 + v2
    v = velocity added to air mass by the prop on treadmill
    v1 = same as v except for wind-machine
    v2 = reduced velocity of air mass on the other side of prop for wind-machine

    In the wind machine, v2 is strictly determined by friction losses in the system: air drag, rolling resistance, and transmission losses. v1 is the only part in the wind machine that adds to vehicle acceleration. Treadmill gets to use the larger v to work totally for acceleration. In the ideal wind-machine v2=0 and it is equal to the treadmill.

    This makes sense because the treadmill is adding energy to the system while a wind machine has to extract it from the difference between the air and ground. Treadmill is adding energy above and beyond the difference engine. Energy and power are come entirely from the wheels on the treadmill, and entirely from the wind on the wind-machine.

    To explain it intuitively: imagine wind car moving 20 mph with no wind. Naturally you use the wheels to turn the prop at a rate so that there is neither drag nor propulsion so that there is overt resistance in wheels or prop and it rolls as far as possible. Then God adds 10 mph tail wind. The prop catches the wind like a sail which adds a force to overcome friction and to accelerate. The part of the force that accelerates the car is immediately transferred to the wheels which immediately accelerates the prop rotation which causes v1 out the back of the vehicle, just like v. Whatever force was lost as friction did not get to cause this chain of events and only increases v1 (slows down the air mass in front of the prop). After working through Dr Bauer and Dr Drela's equations very carefully, I came up with the ratio above.
     
  9. spork

    spork Previous Member

    I can sum this up more succinctly. There is no difference.
     
  10. zawy
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    zawy Junior Member

    It's unfortunate you think an electrically-powered wheel is the same as a wind-powered prop. Energy flows into the prop instead of into the wheel which causes a slight difference. If you were able to do the physics and reviewed the literature, you could see the difference.
     
  11. spork

    spork Previous Member

    It's unfortunate that you don't see what Galileo, Newton, and Einstein saw - that all inertial reference frames are equal. And that it doesn't (and can't) make any difference what caused or causes the difference in velocity between wind and surface - that's all the cart experiences after all.

    If you understood that energy is all about bookkeeping and that you could choose either reference frame for either case, you wouldn't make such silly statements.

    I did the physics and WROTE the literature. There is no difference. Let me know if you want to understand it too. I'll be happy to explain it.
     
  12. zawy
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    zawy Junior Member

    The treadmill motor adds kinetic energy to the air mass. When operating in the wind, kinetic energy is taken out of the air mass. Einstein believed in thermodynamics, too. The direction of energy and power flow is not relative except with respect to time, and only then if it is completely reversible, but in this case it is not. As I mentioned, the 2 cases are exactly equal if there is no friction, so that then a relative point of view can be applied.
     
  13. kerosene
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    kerosene Senior Member

    Nope. "The treadmill motor adds kinetic energy to the air mass." -this is not very clear but I think I know what you are after.

    regardless - such thing as kinetic energy, speed etc. are all relative to reference point. You have no potential energy sitting on flat ground as you cannot fall anywhere. However if I dug up a 10 feet deep hole next to you - you would have potential energy that I could put in use by pushing you into the hole. All that changed was the reference point of "zero" height.

    Likewise in both cases - cart on treadmill in still air or cart in still ground in moving air - the cart is slowing down the air in relation the ground.
    Yes in treadmill case it accelerates it in relation to the room maybe - but "room" is absolutely irrelevant for the cart.

    Only what the cart experiences is relevant.

    Nature creating wind or electric motor moving the ground makes no diff. Both are external forces that obviously create the condition we can harvest the speed from. But the cart doesn't care which one is moving in relation to the spectator.

    edits: typos
     
  14. ThinAirDesigns
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    ThinAirDesigns Senior Member

    No.

    JB
     

  15. zawy
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    zawy Junior Member

    The direction of energy and power flow is relative with respect to time *only* if it is completely reversible. If there is friction anywhere in the system then it is not reversible. As I mentioned, the 2 cases are exactly equal if there is no friction, which is when treadmill applies exactly to the case of the wind.
     
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